U.S. Pat. No. 3,702,951 to Kosmahl incorporated herein by reference discloses a high efficiency multistage depressed collector for use with a source of charged particles such as spent electrons. Each electrode plate of the collector has a central aperture with the apertures increasing in diameter in a downstream direction from the source of charged particles. The electrode plates are generally bowed towards the source of charged particles with the final electrode plate being conical and having a spike extending toward the source of charged particles.
U.S. Pat. No. 3,764,850 to Kosmahl also incorporated herein by reference discloses an electron beam controller in which a magnetic field of predetermined intensity and shape is applied to a spent electron beam to refocus the spent electrons prior to their injection into a collector.
U.S. Pat. No. 3,463,959 to H. R. Jory et al discloses a charged particle accelerator apparatus including means for converting a rotating helical beam of charged particles having axial motion into a non-rotating beam of charged particles. This is accomplished by utilizing crossed electric and magnetic fields. The energy of the beam is not changed while the type of motion is substantially altered.
U.S. Pat. No. 4,199,709 to Jean-Louis Alirot et al discloses an injector for an annular beam of monokinetic electrons in helical orbits having a high inclination angle relative to the axis of the helix of the type having an annular electron gun in a revolving vacuum enclosure.
None of the above patents are concerned with converting the azmuthal energy of spiralling electrons into longitudinal kinetic energy and injecting the electrons into a collector at high efficiency.